Heating with logs is generally considered a rural occupation for perhaps obvious reasons, but there are log boilers, and even log-burning stoves, that deal quite comfortably with smokeless zones. It is often a matter of the practicalities of using logs rather than regulation which decides the matter and that often hinges on the volume involved.
A tonne of mixed hardwood and softwood logs, seasoned for at least two years, will deliver around 2,500kWh of heat. Compare that to the amount of heat the house consumes (or will consume) and you can calculate the number of tonnes of logs needed. A tonne of close-stacked logs will occupy around 1.5 m3. Now work out how much space you will need to store a year’s supply of logs, then double it.
Most logs are advertised as ‘well seasoned’ but typically they’re not. It is important that the moisture content is down around 20% – certainly below 30% – to enable them to burn efficiently. Wetter logs will provide less heat and leave more deposits in the combustion chamber and flue. So buy next year’s supply this year to give it another year to dry.
The table below shows the average UK prices for various fuels (courtesy of the Energy Saving Trust). By comparison, logs cost from zero, if you have your own trees, to 2.6p per kWh, if you buy seasoned mixed hard and softwood logs. At worst case it is almost half the price of gas, but it’s a much greener fuel.
|Carbon Dioxide Factor
Perhaps more importantly, according to a Government briefing paper published in July 2012, gas has risen in price by 143% since 2004; heating oil has risen from 10p per kWh in 1999 to 61.4p in 2012. In a similar period the average price of logs has not changed.
How Do I Do It?
The log-burning stove with back boiler may be the answer, but there are some issues. Firstly, back boilers are said to reduce the efficiency of the stove. That can be true but with a fuel cost between zero and 2.6p per kWh, does it really matter? More important is the actual output. A stove boiler will be rated at, say, 12kW with 4kW to the room side and 8kW to boiler side. Those figures will be accurate when the stove is full of well-seasoned hardwood and the fire is roaring. At all other times the actual output will be something less.
It is also necessary to consider how much heat the room that contains the stove boiler needs. A 20m2 room insulated to Building Regulations standards is likely to need less than 2kW to heat it. That means the stove boiler will maintain comfort at less than 50% output with less than 4kW being delivered to the boiler side.
As a consequence it is very difficult to calculate accurately the size of stove boiler needed in a mixed system — where a log burner is working with a gas or oil-fired boiler. It is, essentially, guesswork. The answer is generally to install a stove boiler where 60% to 70% of the rated output is sufficient to heat the room, and live with whatever boiler-side output that gives.
The second issue is that solid fuel boilers are governed by HETAS regulations and cannot be connected to unvented hot water cylinders, except where specific provision is made. A few stove boilers have the appropriate controls on board, as do a couple of unvented cylinders. It is necessary for the owner to check that the whole system (stove boiler, pipework and cylinder) properly complies with HETAS regulations. The reason being that a log fire cannot be switched off when the cylinder is up to temperature and there is a risk of boiling the cylinder — resulting in a big, wet bang.
Installing a log boiler is a good deal easier in that the cylinder will be matched to the boiler and to the demands of the house. But the cylinder will generally be a good deal bigger than usual – upwards of 1,000 litres – to ensure sufficient energy is stored from a single ‘burn’ to meet the demands of the house for a day. Most log boilers are batch boilers, in that a single batch of logs is loaded each day and that batch is sufficient to heat the cylinder.
There are a plethora of batch log boilers to choose from, varying hugely in terms of outputs, price and level of sophistication. The amount of work needed is reduced in that loading the boiler is a once-per-day activity taking maybe 10 minutes whereas a log-burning stove will need feeding through out the day.
The cost of a stove boiler will vary from £500 to £5,000 with quality and sophistication. A batch log boiler will cost from £3,000 to £15,000 with quality and size. Which sounds like a lot of money but these are big, heavy machines with none of the things that tend to wear out on a gas or oil boiler. Consequently they have a life expectancy of over 20 years and with good maintenance, up to 30 years.
What to Do in Summer?
If logs – either a stove boiler or batch log boiler – are the principal heat source then summer can be a problem. Integrating with a solar thermal panel will deliver the summer load, allow ‘tick-over’ heating for periods when the house is empty in winter and makes a lot of sense.
The Least You Should Know
- A log-burning stove with back boiler is theoretically capable of heating the average home and providing ample hot water.
- Most solid fuel boilers cannot be connected to unvented (sealed) hot water systems.